An optical phase plate is hereinafter understood to be an optical element which is permeable to light and imparts a phase offset to a light beam passing therethrough. The phase offset is dependent upon the location of the pass-through of the light beam through the optical element.
Optical imaging systems (for example, in cameras or microscopes) image an object region into an image plane via lens systems. As a rule, in such apparatus, the position of the image plane with reference to a lens system is constructively pre given and cannot be adjusted. Accordingly, for example, in a camera, a light-sensitive film is disposed in the image plane. The position of the film is fixed relative to the lens system in the camera. In a microscope, the position of the image plane is determined by a corresponding dimensioning of the microscope main objective and tube lenses and is, as a rule, likewise not adjustable.
Strictly speaking, in the model of the geometric optics, only such object points are sharply imaged from which light rays emanate which are so deflected by the lens system in the corresponding apparatus that these rays all intersect in a common image plane. Taken precisely, this pertains only to specific object points in an object plane. The position of these object points with reference to a primary plane of the lens arrangement is determined by the position of the image plane and the dimensioning of the lens system.
In the model of wave optics, an individual object point leads to a brightness distribution with finite expansion in the image plane. This image of the object points in the image plane is therefore basically limited with respect to diffraction. The form of the brightness distribution of an individual object point for a fixed image plane is dependent especially on the position of the object point with respect to an ideal object plane. For object points at a distance from the ideal object plane, there results a more expanded brightness distribution in the object plane than for object points which lie in the ideal object plane. The expansion of the brightness distribution can be reduced in that the aperture angle of the beam, which originates from an object point and impinges on the image plane, is reduced. This can be especially obtained by shading the geometric beam path. A shading of the geometric beam path leads, however, to a loss of image brightness. In this way, the depth of field of the optical image system can be improved because, in this way, also object points, which lie outside of an ideal object plane, are imaged with a comparatively low expanded brightness distribution. By shading the beam path, it is thereby possible to generate an image in the image plane which is only diffraction limited. The increase in depth of field achieved in this way can be estimated with the following formula:Δz=C×λ/[NA]2,wherein: C is an apparatus constant of the optical imaging system; λ is the wavelength of the imaging light; and, NA is the numerical aperture of the imaging system.
It is known to introduce an apodizing mask into the beam path to improve the depth of field of an optical imaging system. Such an apodizing mask is a diaphragm having a diaphragm function which continuously changes in a peripheral region of the beam throughput between “throughput” and “total shading”. The apodizing mask causes that the diffraction image, which is caused by an object point in the image plane, has an expanded primary maximum. At the same time, in this diffraction image, the secondary maxima are, however, formed with a corresponding lesser intensity. For object points, which are disposed in an ideal object plane, resolution capacity is thereby increased. At the same time, however, object points, which are disposed in the proximity of one such object plane, are imaged comparatively sharper. As for a simple shading of a beam path, with the use of an apodizing mask, the increase in depth of field is, however, obtained with a loss of image brightness.
The depth of field of optical imaging systems can be increased also by means of dynamic scan methods. For this purpose, an object region is digitally evaluated at different adjustments of the imaging system. In each case, the adjustment corresponds to a sharp imaging of different object planes. The detected image signals are then superposed to a depth-sharp total image. For a flicker-free image presentation, a very rapid data processing is, however, needed. The complexity with respect to computation is then very high for high image resolution.
An optical imaging system is described in the article of Dowski, Jr., et al entitled “Extended depth of field through wave-front coding”, Applied Optics, volume 34 (1995), page 1859, and in U.S. Pat. No. 5,748,371. In this imaging system, an object region is imaged by means of a lens onto a CCD camera. At the object end, an optical phase plate is assigned to the lens. This optical phase plate is configured as a cubic phase plate. A spatially varying phase function is impressed upon the light passing through the cubic phase plate. This phase function causes a completely non-sharp object image in the image plane. This unsharp object image is detected by means of the CCD camera and is supplied in digital form to a computing unit for evaluation. In the computing unit, a depth-sharp image of the object region is computed from the imaged signals, which are detected by means of the CCD camera, and a known optical transfer function of the system. Here, it is utilized that the optical transfer function of the system has proven to be invariant to a good approximation relative to an offset of an object point of an image plane. The offset satisfies the geometric imaging conditions with respect to the image plane in which the CCD camera is mounted.
In U.S. Pat. No. 6,536,898 it is suggested to mount a phase plate on the human eye in order to so adjust the optical transfer function of this optical system that this transfer function is constant over a specific range away from the object. U.S. Pat. No. 6,536,898 explains in this context that a phase plate is realized via a suitable structuring of the cornea or of a contact lens. Alternatively, it is suggested in this U.S. Pat. No. 6,536,898 to implant a corresponding phase plate as an intraocular implant in the human eye.
An increase of the depth of field is desirable especially in surgical microscopes because the tissue, which is operated on by a surgeon, is, as a rule, filled with fissures. Here, it is therefore sought to make possible a sharp viewing for all regions of a surgical area. This is especially of significance in surgery on the eye because here several transparent tissues are present with the cornea, the pupil and the lens which lie one upon the other.